The present invention relates generally to an exposure apparatus, and more particularly to a reflection type or catoptric projection optical system, and an exposure apparatus using the same which use ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light to expose an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”).
Recent demands for smaller and lower profile electronic devices have increasingly demanded finer semiconductor devices to be mounted onto these electronic devices. For example, the design rule for mask patterns has required that an image with a size of a line and space (“L & S”) of less than 0.1 μm be extensively formed. It is expected to require circuit patterns of less than 80 nm in the near future. L & S denotes an image projected onto a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus as a typical exposure apparatus for fabricating semiconductor devices includes a projection optical system for exposing a pattern on a mask or a reticle, onto a wafer. The following equation defines the resolution R of the projection exposure apparatus (i.e., a minimum size for a precise image transfer) where λ is a light-source wavelength and NA is a numerical aperture of the projection optical system:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            
As the shorter the wavelength becomes and the higher the NA increases, the higher or finer the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light narrows usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflective elements, i.e., mirrors instead of refractive elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system cannot include any lenses. It has thus been proposed to form a catoptric projection optical system only with mirrors (e.g., multilayer mirrors).
A mirror in a catoptric reduction projection optical system forms a multilayer coating to enhance reflected light and increase reflectance. The multilayer mirror is characterized in that when it is optimized so as to provide high reflectance to light at a small incident angle, it can provide high reflectance to high at a large incident angle distribution, whereas when it is optimized so as to provide high reflectance to light at a large incident angle, it cannot provide high reflectance to only light at a small incident angle distribution.
More specifically, a multilayer mirror including 40 layers of molybdenum and silicon at a uniform period has an incident-angle range for reflectance of 60% or greater of 0° to 13° when the multilayer mirror is optimized to the incident angle of 0°, and 10° to 17° when the multilayer mirror is optimized to the incident angle of 15°. A multilayer coating with a complex structured, such as a graded multilayer coating that modulates a period of the multilayer according to positions, needs for a large incident angle distribution.
The smaller number of mirrors is desirable to increase reflectance for the entire optical system. In addition, the projection optical system preferably uses the even number of mirrors to avoid mechanical interference between a mask and wafer by arranging them at opposite sides with respect to a pupil.
As the EUV exposure apparatus has requires a smaller critical dimension or resolution than a conventional one, higher NA is necessary (e.g., up to 0.2 for a wavelength of 13.4 nm). Nevertheless, conventional three or four mirrors have a difficulty in reducing wave front aberration. Accordingly, the increased number of mirrors, such as six, as well as use of an aspheric mirror, is needed so as to increase the degree of freedom in correcting the wave front aberration. Hereinafter, such an optical system is referred to as a six-mirror system in the instant application. The six-mirror system has been disclosed, for example, in U.S. Pat. No. 6,033,079, and International Publication No. WO 02/056114A2.
U.S. Pat. No. 6,033,079 discloses two typical six-mirror catoptric projection optical systems, which receive light from the object surface, form an intermediate image via first to fourth reflective surfaces, and re-form the intermediate image on an image surface via a convex fifth reflective surface and a concave sixth reflective surface. Such a structure contributes to high NA by enlarging and introducing light to sixth reflective surface and condensing the entire light on the image surface. Thus, the sixth reflective surface has a large effective diameter. The intermediate image should be formed after the fourth reflective surface to introduce the light into the fifth reflective surface while preventing the sixth reflective surface from shielding the light.
In this case, divergent light enters the fifth reflective surface, increasing the incident-angle distribution on the fifth reflective surface. The first embodiment discloses an optical system that has an arc-shaped image with a width of 1 mm and NA=0.25, and the maximum incident angle is 17.1° and the minimum incident angle is 0.4° on the fifth reflective surface. Therefore, the incident-angle distribution is 16.7°.
As a consequence, a distribution between the maximum and minimum incident angles on the fifth reflective surface significantly deteriorates the reflectance and lowers the throughput on the above multilayer coating.
On the other hand, International Publication No. WO 02/056114A2 also discloses a six-mirror catoptric projection optical system. Different from ones disclosed in U.S. Pat. No. 6,033,079, this catoptric projection optical system forms an intermediate image after the second reflective surface and introduces roughly collimated light into the fifth reflective surface. This catoptric projection optical system somewhat improves an incident angle by introducing collimated light into the fifth reflective surface. For example, for an arc-shaped field with NA=0.25 and a width of 2 mm, the maximum incident angle is 17° and the minimum incident angle is 5.5° on the fifth reflective surface. Therefore, the incident-angle distribution is 11.4°.
Still, this incident-angle distribution is not sufficiently small, and the deteriorated reflectance on the fifth reflective surface lowers throughput. In addition, the first concave reflective surface increases an angle between exit light from the first reflective surface and the optical axis, causing the third and fourth reflective surfaces to have extremely large effective diameters. In particular, an effective diameter of the fourth reflective surface is assumed to be 650 mm when NA is made 0.25, and thus is not viable due to the large size of the apparatus and difficult processing measurements.
In view of the current inefficient EUV light source, high throughput needs an improvement of the deteriorated reflectance on the fifth reflective surface.